This invention relates to acicular bodies of inorganic materials and a method for producing the same. More particularly, this invention relates to micron-sized acicular bodies of metal oxides that are useful as reinforcements for ceramic composites and a method for producing the same. This invention also relates to composite materials having acicular bodies of metal oxides as reinforcements.
Shaped bodies of ceramics have found uses in many applications such as aeronautics, medical diagnostic systems, energy conversion, automotive components, and lighting. In some of these applications, the ceramic bodies are typically sintered shaped bodies that are resistant to deterioration in a harsh environment, such as high-temperature or corrosive environment. In other applications, such as in medical diagnostic systems, sintered bodies of certain ceramics, such as x-ray scintillators, are used for the unique properties of the chosen ceramic to luminesce upon being excited by a stimulating radiation. However, sintered ceramic bodies typically have smaller tensile stress than compression stress and are prone to cracking. To impart increased toughness and fracture resistance, these bodies are often reinforced with other inorganic fibers, such as metal carbide, boride, nitride, or oxide.
One common method for producing inorganic fibers is disclosed in U.S. Pat. No. 5,686,368. In this process a fibrous metal oxide product is made by providing an fiber template made of an organic material such as polyester, rayon, cellulose, etc.; soaking the fiber template to impregnate it with a rare-earth nitrate; drying the impregnated fiber; and heating the impregnated fiber to burn out the organic template and to convert the rare-earth metal nitrate to rare-earth metal oxide. The resulting product is a rare-earth metal oxide fiber having substantially the same shape and dimension as the organic fiber template.
U.S. Pat. No. 5,865,922 discloses a similar process for making ceramic fibers. In this process, fibers of a polymeric material are coated with silicon carbide or nitride vapor at a very high temperature to yield a partially rigid fibrous body. The coated fibers are infiltrated with an organic resin material, then pyrolyzed to produce fibers comprising porous carbon and silicon carbide or nitride. The porous fibers are then infiltrated with liquid silicon to react with the porous carbon to yield fiber predominantly composed of silicon carbide.
In another process disclosed in U.S. Pat. No. 6,120,840; a fibrous material is infiltrated with a polymer precursor of an organic transition metal complex. The polymer precursor is then cured and decomposed to convert the organic transition metal complex to a transition metal boride or carbide.
Although good inorganic fibers could be obtained from these processes, the manufacturing cost is undoubtedly high due to the use of possibly costly organic fiber raw materials and the complex multi-step nature of the processes.
Therefore, there is a need for a simple process for making inorganic fibers. It is also very desirable to produce inexpensive inorganic fibers without resorting to using organic fiber templates. Furthermore, in many instances, it is very desirable directly to produce inorganic fibers that have the same composition as the ceramic matrix in which they will reside.
The present invention provides inorganic acicular bodies comprising at least one inorganic compound of a metal selected from the group consisting of Groups IA, IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, VB, VIA, VIB, VIIA, VIIB, VIIIA, rare-earth metals of the Periodic Table, and mixtures thereof. These acicular bodies may be used in reinforcement of composite ceramic bodies. The inorganic acicular bodies may be produced directly to have the same composition as the ceramic bodies. More particular, the inorganic acicular bodies comprise metal oxides and have a cross-sectional dimension of less than about 20 xcexcm. The inorganic acicular bodies are characterized in that their cross section is generally polygonal and their lengths are much longer than their cross-sectional dimension.
According to one aspect of the present invention, the inorganic acicular bodies are produced by a method comprising the steps of: preparing a solution of a precursor of the inorganic material; adding the solution of the inorganic material into a solution of an ester of a dicarboxylic acid; precipitating an organic salt of the ester of the dicarboxylic acid comprising the inorganic material (hereinafter called the organic salt) in acicular shape; separating the acicular-shaped bodies of the organic salt; drying the acicular-shaped bodies of the organic salt; and firing the acicular-shaped organic salt in an oxidizing atmosphere to produce inorganic acicular bodies.
Other aspects, advantages, and salient features of the present invention will become apparent from a perusal of the following detailed description , which, when taken in conjunction with the accompanying figures, discloses embodiments of the present invention.